The present invention relates to the manufacture of ceramic honeycomb structures of the kinds used for particulate filters, catalyst supports, and heat exchangers, and in particular to the manufacture of ceramic honeycombs exhibiting improved resistance to water absorption in the unfired state, and thus increased structural integrity when contacted with aqueous media.
Honeycomb structures having transverse cross-sectional channel or cellular densities of approximately 1 to 200 cells or more per square centimeter have been adapted to a number of uses, including use as solid particulate wall-flow filter bodies, catalyst supports, and stationary heat exchangers. For wall-flow filter applications the honeycomb structures are conventionally provided with channel-blocking plugs or seals at opposite ends of alternate channels, thereby establishing filtration flowpaths that traverse the porous ceramic walls of the structures and thus provide for the efficient trapping of particulates within the honeycomb structure. Unplugged catalyst supports and heat exchangers are also in widespread commercial use.
As illustrated in the drawings, reference numeral 10 (FIG. 1) generally designates a solid porous ceramic honeycomb body of conventional type that is typically fabricated via well known honeycomb extrusion and plugging processes. Body 10 includes a honeycomb core structure 12 formed by a matrix of intersecting, thin, porous walls 14, and it may further comprise and extruded or after-applied outer wall or skin 15. The walls 14 extend across and between a first end 13 that includes a first end face 18, and a second end 17 that includes an opposing second end face 20, walls 14 thus forming a large number of adjoining hollow cells or channels 22 which extend between and are open at the end faces 18, 20 of the filter body 10.
To form a filter from this honeycomb structure 10 (FIGS. 2 and 3), one end of each of the cells 22 is sealed, a first subset 24 of the cells 22 being sealed at the first end face 18, and a second subset 26 of the cells 22 being sealed at the second end face 20. Either of the end faces 18, 20 may be used as the inlet face of resulting filter 10.
A typical method for manufacturing the honeycomb structure 12 described above includes the steps of compounding a powder batch mixture comprising inorganic powders, an aqueous vehicle, and one or more organic cross-linkable batch constituents such as organic binders, lubricants and plasticizers, and thereafter forming the batch mixture, e.g. by extrusion through a honeycomb die, to form a honeycomb extrudate. This extrudate is then dried, cut and fired to sinter or reaction sinter the inorganic powders into unitary ceramic structures of honeycomb configuration. Plugging of the honeycombs to produce wall flow filter bodies may be carried out before or after drying or firing.
Depending upon the design of the honeycomb extrusion dies and the honeycomb forming process, honeycomb structures produced as above described may be directly extruded with integral skin layers, or they may be formed as honeycomb core bodies without permanent skin layers. In the latter case, a relatively thick, after-applied surrounding skin 57 (FIG. 4) is generally provided on the core bodies. Such skins are typically provided through a subsequent coating or wrapping of the cores with a powder batch mixture like that used to form the honeycomb core bodies, which is then dried. The process of coating or “skinning” can be carried out after the formed honeycomb core bodies have been dried, or even after they have been fired.
For many applications, honeycombs fabricated as above described must be subjected to supplemental processing with aqueous media, for example, to deposit washcoats or catalyst coatings thereon or to apply passivating pre-coatings or cover coatings to protect or condition the honeycombs for further processing or use. Generally, such processing is not harmful if the honeycombs and/or skin layers have been fully fired. However, if the honeycomb core, or skin, or both have not been fired, then the unfired components of the honeycomb still retain moisture and batch conditioning additives such as cellulosic binders that are prone to the absorption of water during further processing. Such absorption can be particularly problematic to the extent that it causes swelling or structural degradation of the honeycomb structure or honeycomb skin layers.
For the mass production of such honeycomb products, it is highly desirable to be able to reduce the associated cycle time for production as much as possible, while maintaining certain quality standards in the resultant filters. Thus methods for manufacturing honeycomb structures for use as filters, flow-through catalyst supports, or heat exchangers that are compatible with the application of washcoats, catalyst coatings or other coatings from aqueous media while maintaining or improving the overall structural integrity of the resultant products would offer substantial production cost and quality advantages.